This invention relates to a connection, including a single-piece, in-line sheet metal holdown strap connector, for attaching the vertical frame member of a structure to a concrete foundation.
Earthquakes, hurricanes, tornadoes, floods, or tidal action impose upward forces on a building structure that can lift it off its foundation. To counteract these uplift forces, it has become standard practice to tie the structure to its concrete foundation. The oldest and probably still the most common practice of making this connection is to attach the framed structure to threaded anchor bolts embedded in the concrete foundation. How the connection is made to the framed structure is important in determining the resistance of the connection to uplift forces. The threaded anchor bolt can attach to the framed structure at the mudsill member of the framed structure, however, the connection is more effective if the anchor member is attached directly to the vertical frame members or studs of the structure.
One approach adopted by prior art for connecting the embedded anchor to the vertical frame members has been to use a separate metal holdown connector to connect the threaded anchor to the vertical frame member. U.S. Pat. No. 5,249,404, granted Oct. 5, 1993 to William F. Leek and Alfred D. Commins, represents the state of the art of a connection between a concrete structure and a vertical frame member, using a threaded anchor bolt and a separate metal holdown connector. Adding a metal holdown connector integrated well with the existing method of tying the threaded anchor bolt to a mudsill, however, the use of multiple parts for the connection is both labor intensive and relatively expensive.
Also, metal holdowns used in these connections must be designed to resist the moment force of the uplift force that is inherent in their design. The attachment interface of the metal holdown to the vertical frame member and the attachment interface of the metal holdown connector to the threaded anchor bolt are not substantially in line, thus the uplift force exerted on the connection also has a significant moment force that the metal holdown connector must bear.
Another approach in the prior art to making this connection is represented by disclosures of holdown strap connectors. In this method, the threaded anchor bolt is replaced by an embedded strap that passes around the outside of the mudsill member rather than through it, and attaches only at the vertical frame member. U.S. Pat. No. 5, 150,553, granted Sep. 29, 1992 to Alfred D. Commins and Ernest A. Romeo represents the state of the art of a connection between a concrete structure and a vertical frame member with a holdown anchor strap connector. Anchor straps are relatively inexpensive both in terms of material costs and installation costs and are thus attractive to developers and builders. Because anchor straps circumvent the mudsill rather than going through it to attach to the vertical frame member, the step of drilling the mudsill is eliminated. However, by going around the mudsill (usually on the outside), the anchor strap can interfere with other steps in the building process, such as the attachment of side paneling or plywood shear walls. U.S. Pat. No. 5,150,553 improved on prior anchor straps by attaching to the side face of the vertical frame member rather than to its outside edge. Attaching to the side face left more of the outside edge of the vertical frame member exposed and available for the attachment of siding or plywood shear wall panels. Attaching to the side face also allows fasteners to penetrate the wide face of the vertical frame member thus reducing splitting when wood frame members or studs are used. However, with anchor strap connectors an issue that was relatively minor for connections using threaded anchor bolts becomes very important, and that is proper placement of the embedded portion for insuring the achievement of the necessary design resistance load values.
The resistance of embedded anchors in concrete to uplift forces decreases as the anchor's distance from the sides and edge of the concrete structure decreases. Thus, maximizing the concrete surrounding the anchor will increase the anchor's resistance to uplift forces.
Current methods of building framed structure limit the options a connection designer has for increasing the horizontal distance between the anchor and the outer or inner side faces of a typical 6" concrete stem wall. Generally, the mudsill member is placed so that its outside edge is flush with the outside edge of the concrete foundation. A threaded anchor bolt that pierces this mudsill should thus be placed near the center of the mudsill member to increase its hold on the member. An anchor strap that goes around the outside of the mudsill is similarly limited in where it can protrude from the concrete structure in relation to the horizontal distance from the outside edge.
When the vertical frame member to be anchored forms part of a corner or the end of a shear wall then the distance from the embedded anchor to this corner edge becomes important. Under current practice, a threaded anchor bolt will always have at least the width of the vertical frame member between itself and the corner edge of the concrete. Anchor straps that attach to the outside face of vertical frame member, however, have no similar minimum distance for this measurement determined by the design of the shear wall to which the anchor will attach. Were a builder to disregard the need to maximize this distance between the corner edge of the concrete structure and the anchor strap connector by placing the edge of the anchor strap connector at the edge of the concrete foundation the practical utility of the anchor strap connector would be largely defeated. It should also be noted that anchor strap connectors that attach to the outside edge of the vertical frame member can never be very far from the corner edge of the concrete foundation, since they commonly use much of the width of the outside edge of the vertical framing member for attachment. In comparison, the prior art's anchor strap connectors that attach to the side face of vertical frame member have at least the width of the vertical frame member between the attachment portion of the anchor strap connector and the corner edge, however, depending on the design of the anchor strap connector, the distance from the embedded portion of the anchor strap connector to the corner edge can be more or less than this distance. With certain designs of anchor strap connector that attach to the side face of the vertical frame member this distance also depends on the chirality of the corner where the anchor strap connector is placed.
For example, the one-piece version of U.S. Pat. No. 5,150,553 has different load capacities at otherwise uniform left and right corners, because of the different orientation of the embedded strap portion to the side face of the vertical frame member and thus to the corner edge of the concrete foundation as well. This problem is solved by making and using both right and left chiral versions of the one-piece design. However, having both right and left forms is confusing for the builder, and means added manufacturing costs.
On the other hand, the two-piece version of U.S. Pat. No. 5,150,553 attaches to a vertical framing member at similarly designed left or right corners with generally the same distance between the near edge of the embedded portion of the anchor strap connector and the corner edge of the concrete foundation. Thus if standard construction practices are used and the directions for installation are followed, the distance between the embedded portion of the anchor strap connector and corner edge, depends only upon the design of the vertical frame member that constitute the corner or edge. However, instructions for the installation of the two-piece connector of U.S. Pat. No. 5,150,553 are relatively complicated. Also the two-piece design that allows for similar placement at right and left corners uses an elongated strap portion with tabs that protrude past the outside edge of the vertical frame member. When plywood shear walls are attached over the two-piece connector of U.S. Pat. No. 5,150,553 there is greater bulging of the plywood then when the one-piece version is used.
Both versions of U.S. Pat. No. 5,150,553 suffer from complicated requirements for their placement in the concrete foundation. The builder pouring the concrete foundation and setting the holdown anchor strap connector needs to first mark where the side face of the vertical member will be on the concrete form board. He then attaches the holdown strap connector to the concrete form board to the left or right of this mark a selected distance based on his knowledge of whether the embedded strap portion and transition member is to be substantially to the left or right of the side face of the vertical frame member. With the one-piece version of U.S. Pat. No. 5,150,553 the builder must also keep track of which chiral version of the holdown strap connector is to be used. Proper placement is thus dependent on the builder not only knowing where the side face of vertical frame member will be, but the orientation of the holdown strap connector to that side face.
The one-piece version of U.S. Pat. No. 5,150,553 also does not achieve substantial alignment of the connections between the vertical frame member and the concrete foundation, thus the uplift force exerted on the connection has a significant moment force that the metal holdown connector must bear.